1. Field
The following description relates to a packet or passive optical network, and more particularly, to a packet or passive optical network capable of packet-based protection switching against passive optical network (PON) failures.
2. Description of the Related Art
So far, various forms of passive optical network (PON) protection switching architectures and mechanisms have been suggested, such as the PON protection switch and protection switching method disclosed in US Patent Application No. 2010/0183298 discloses such a. However, the existing protection switching architectures have a range of problems. For example, the protection switching architecture may be costly or may not address inherent limitations of the PON. In addition, the application of the conventional protection switching architecture to the presently popular time-division-multiple-access-PON (TDMA-PON), such as an Ethernet PON (EPON) and Gigabit-capable PON (GPON) may not be possible; also some protection switching architectures can only deal with failures in particular segments of a network, making them inapplicable to a system that has to be capable of handling failures in the entire network.
In recent years, the application of PON has been widened to broadband access networks for various services, such as wired and wireless convergence. Accordingly the importance of protection switching is increasing. Regarding PON protection switching, ITU-T G. 983.1 Appendix IV describes four mechanisms for forming optical lines in a PON system, namely, fiber duplex, OLT-PON duplex, full duplex, and partial duplex, but does not specify a protection switching mechanism and apparatus that utilizes the optical lines.
Particularly, protection switching in TDMA-PON has distinctive characteristics, compared to general network equipment. First, TDMA-PON protection switching is performed characteristically between network equipment and a general subscriber device, not between pieces of network equipment. Due to cost incurred to perform protection switching, some subscribers may not want protection switching.
For instance, some subscribers may only want a level of protection switching that allows for losses to occur so long as the amount does not cause service interruption, while other subscribers may want complete lossless protection switching. Therefore, only a system with protection switching architecture and functions that are responsive to various needs of subscribers can be employed as a universal protection switching mechanism. At this time, the minimization of cost incurred from protection switching should be considered.
Second, the PON architecture itself is the characteristic feature, in that it selectively performs protection switching that is optimal to a specific segment at which a failure has occurred in the PON system can reduce the influence of the failure and incur less implementation cost. Hence, when compared to general protection switching for communication devices, TDMA-PON protection switching cannot be implemented using only one protection switching mechanism. In a TDMA-PON protection switching architecture, different protection switching functions are mixed, and thus the different protection switching functions need to be allowed to interact with each other.
Third, the TDMA-PON protection switching is related to packet services. For packet services, packet paths are determined by autonomous functions (routing, switching, etc.) of communication devices, such as a router and a switch. In the event of a failure, the failure is reflected in autonomous functions and, in turn, a new packet path is determined. In this case, it takes a significant length of time for the system to acclimatize to the new path after the failure, and data loss and duplication cannot be avoided.
Hence, improving a protocol to adapt promptly to network changes can be one method for resolving aforementioned issues of data loss and duplication, but a more reliable method for protection switching in a packet network would be to designate a protection switching path beforehand, detect any failures using a network management control function or a network's autonomous function, and quickly switch the path to a protecting switching path upon detection of a failure. Although PON link segment of TDMA-PON, itself, does not exhibit such packet characteristics, packet protection switching characteristics cannot be ignored in this segment since the next upper layer generally functions as L2/L3 and protection switching is terminated in such a layer.
As related arts, both U.S. Pat. Nos. 7,843,933 and 8,144,711 each disclose that identical packets are sent through a plurality of packet paths and a receiver terminal selects one path from the multiple packet paths, thus receiving a normal packet; in the event of a failure on the selected path, the receiver terminal changes the receiving path from the selected path to another path among the multiple paths in order to receive a normal packet.
Both related arts suggest the operations to be performed based on packet sequence numbers. That is, a transmitter side sends a packet with a sequence number attached thereto, and a receiver side confirms receiving the identical packets based on the sequence numbers and performs filtering. This method may be advantageous in a case where the sequence numbers can be attached to packets, but in the case of PON, an additional field has to be defined to assign a sequence number to an Ethernet frame.
As described above, PON protection switching mechanisms to date focus on only some functions, and thus it is difficult to find an optimal mechanism that satisfies the aforementioned TDMA-PON protection switching characteristics/requirements, and the application of the previously suggested packet protection switching mechanism to the PON is not easy. Therefore, the applicant suggests a packet-based PON protection switching mechanism which is provided by applying the packet protection switching to a PON system so as to improve the existing packet protection switching mechanism and thus satisfy the TDMA-PON protection switching characteristics/requirements.